In the manufacture of semiconductor devices, ion implantation is used to dope a workpiece, typically provided in the form of a substrate such as a Silicon or Gallium Arsenide wafer. The wafer is bombarded with impurities or dopants for implanting these dopants within the crystalline structure of the wafer to modify the electrical characteristics or otherwise transform the substrate. As such, ion implantation systems are well-known in the semiconductor manufacturing field, as capital equipment utilized to dope workpieces with ions by implanting ions from an ion beam into the workpiece, or to form passivation layers during fabrication of an integrated circuit. When used for doping semiconductor wafers, the ion implantation system injects a selected ion species into the workpiece to produce the desired extrinsic material.
Typical ion implantation systems include an ion source for generating electrically charged ions from ionizable source materials. The generated ions are formed into a beam and accelerated with the help of a strong electric field so as to be directed along a predetermined beam path to an implantation end station. For example, implanting ions generated from source materials such as antimony, arsenic, or phosphorus, for example, results in an “n-type” extrinsic material wafer, whereas a “p-type” extrinsic material wafer often results from ions generated with source materials such as boron, gallium, or indium.
The ion implantation system may include beam forming, steering, deflecting, shaping, filtering and charging subsystems (e.g. beam optical elements or beam optics) positioned between the ion source and the end station. The beam optical elements manipulate and maintain the ion beam along an elongated interior cavity or passageway (e.g beamline) through which the ion beam passes on route to the end station where the workpiece is positioned.
In most ion implantation applications, the goal of the implantation process is to deliver a precisely-controlled amount of dopant uniformly over the entire area of the surface of the workpiece or wafer. The most widely accepted approach for this process is embodied in a so-called serial implantation architecture, where individual workpieces are sequentially provided to an end station for implantation by the ion beam. In order to achieve uniform doping utilizing an on beam having a size that is smaller than the workpiece area, the ion beam and the wafer are moved relative to one another in order to cause the beam to impinge on the entire surface area of the wafer. One commonly known system architecture for accomplishing this task is known as “two dimensional (2-D) mechanical scanning,” as disclosed, for example, in U.S. Pat. No. 6,956,223, where the wafer is scanned in two substantially orthogonal dimensions with respect to a stationary “spot” ion beam. Using 2-D mechanical scanning, the wafer is quickly scanned in front of a fixed ion beam in a so-called “fast scan” direction, while being simultaneously slowly scanned in an orthogonal “slow scan” direction, thereby “painting” the wafer with ions by transporting the wafer in front of the ion beam in a generally moving zigzag pattern. Alternatively, another well-known serial system architecture used in ion implantation systems is a so-called “hybrid scan system,” where the ion beam is swept or scanned back and forth along an axis in one direction in a raster-like manner to form a ribbon-shaped beam, and the workpiece is mechanically moved along a direction orthogonal to the axis of the scanned ion beam.
A continuing trend in the field of semiconductor manufacturing involves various semiconductor workpiece sizes, such as 300 mm diameter wafers, coupled with higher device densities. Larger workpiece size increases the cost of individual workpieces, and higher device densities increase the cost of processing, and the associated value of each workpiece. As a result, control over implantation uniformity with respect to ion beams and other parameters is more critical than ever in avoiding or mitigating the cost associated with scrapping workpieces.
In order to maintain uniformity of the implantation process, the total ion beam current is often measured during implantation, wherein a sampling cup, typically a Faraday cup or cups, is/are placed along the path of the ion beam, typically in front of, adjacent to, or behind the wafer. As in the case of a scanned beam architecture, the beam scan width is generally dictated by the position of the Faraday cup(s) such that the ion beam is fully, or at least partially, scanned over so-called side cup(s) adjacent to the wafer adequate to produce reliable beam current measurements. A tuning cup or cups may also be positioned upstream or downstream of the typical location of the workpiece for tuning of the ion beam when the workpiece is not present or is in a position such that at least a portion of the ion beam does not impinge on the wafer. In addition, a traveling, or so-called “profiling” Faraday cup can be used to monitor the ion beam as the Faraday cup is set in motion in front of the workpiece position from one end of the scanned ion beam to the other. The profiling Faraday can be provided in the form of a multi-cup structure or a single multi-Faraday per cup structure. Conventionally, all (or some) of such sampling cups are utilized to monitor the total current of the ion beam entering the end station in order to adjust implant feed rates and exposure times of the workpiece to the ion beam. For example, in U.S. Pat. No. 4,922,106, a Faraday detector is slowly translated to produce an integrated beam current or dose measurement as a function of the position of the Faraday detector, thereby providing a signal representative of the ion beam intensity. This signal may be used to adjust an oscillatory scan voltage so that the integrated beam intensity is uniform. In that patent, a time integral of the sensed beam current is used as a feedback signal that is applied to a dose controller for controlling the operation of the beam scanning element.
One problem, which is presently appreciated, is that beam current density or angle, often changes unpredictably during an implant cycle, sometimes during a single ion beam scan pass, and sometimes multiple times during the course of a single scan pass. These changes can be manifestations of long-term wear of beamline components over time, or drift in voltage power supplies over the course of multiple ion implant cycles, or during a single ion implant cycle. These variations can also be manifestations of short-term fluctuations or “spikes” in current distribution caused by disparities in beam shape and/or angle, shifting of beam distribution within the ion beam, system noise, or particle contamination within the ion beam, among other factors. Other factors can include changes in the beamline pressure, emission from elements exposed to the beam, and beam interaction with the wafer as it is moved through the beam. While some variations in beam current can be expected and tolerated as a so-called predicted nonuniformity (PNU) in the ion beam, many types of changes in beam current can significantly impact ion implant uniformity and cannot be tolerated and are unacceptable.
The present invention addresses these issues by monitoring the ion beam current via high frequency sampling to generate a waveform representative of real time, in situ beam current as the ion beam is scanned. The generated waveform can be stored and/or displayed in real time to provide a visual representation of the ion beam current information in graphic form for beam uniformity. The beam current sampling can also be analyzed and used to generate control signals for controlling the ion implantation system. One exemplary control signal can abort a given implant cycle in cases where the beam current drift, variation and/or fluctuation exceeds a predetermined threshold level relative to previous current samples or when the beam current falls outside of a predicted nonuniformity (PNU) for the beam current. In addition, collected sample data can be used to provide feedback signals for varying at least one of the upstream beam optical elements or power supplies associated therewith to provide a more uniform beam current density delivered to the workpiece.